The World Health Organization has categorized dengue as a high priority emerging viral disease. A fine line exists between protective immunity and pathogenic immune responses that lead to severe dengue hemorrhagic fever (DHF). A better understanding of protective immunity will have a direct impact on dengue vaccine trials and implementation strategies in endemic countries. A better understanding of DHF immunopathogenesis will lead to improved triage, therapeutic, and intervention strategies. The goal of this proposal is to delineate protective and pathogenic human immune responses against dengue virus (DENV) infections in the unique immunological setting of infancy. Studies of primary DENV infections during infancy provide important human immunological data that cannot be obtained from studies of older children or adults. The first aim of this project is to conduct a prospective clinical study of DENV infections during infancy. The study site is in San Pablo, Laguna, Philippines. The prospective study will collect clinical and epidemiological data, anthropometric measurements, and blood samples from infants. Surveillance activities will capture infant DENV infections across the entire spectrum of disease severity, including clinically inapparent infections. The first aim will characterize the wide clinical spectrum of infant dengue, define overall and age-specific incidence rates, and delineate associations between infant age, body fat mass/anthropometric indices, and dengue disease severity. It will also provide crucial pre-infection, post-infection, and longitudinal infant blood samples for the other specific aims. In the second aim, neutralizing antibody titers and anti-DENV E protein domain III IgG levels and avidity will be measured using the pre-infection infant blood samples. Antibody-dependent enhancement (ADE) of DENV infection and anti-DENV prM/anti-E IgG ratios will also be measured. The assays will be conducted so as to reflect circulating antibody levels in infants at the time of their DENV infection. Serotype-specific antibody levels that correlate with in vivo protection against symptomatic dengue will be defined. The proposed ADE assays will provide solid clinical data to either support, refute, or significantly modify the existing ADE dengue pathogenesis paradigm. The third aim will characterize changes in distinct infant innate immune responses with age and body fat mass/nutritional status, and delineate their potential role in shaping dengue disease severity. Innate immune cell activation profiles to specific Toll-like receptor/pattern-recognition receptor agonists and DENV infection will be measured by several methods using peripheral blood mononuclear cells collected longitudinally from infants over ages 2-16 months old. Additional experiments in this aim will focus on identifying specific cellular signaling pathways that contribute to impaired innate immune responses in infants with low body fat stores (i.e. undernourished/malnourished), and subsequently at low risk for developing DHF. This project and innovative clinical study are uniquely poised to address critical gaps in knowledge regarding protective and pathogenic human immune responses in dengue.